It has been observed that when a plant is transformed with a gene encoding a viral coat protein (CP), the resulting transgenic plant is often less susceptible or resistant to infection by that same virus. This phenomenon has been referred to as "coat protein-mediated resistance" (See Beachy et al, 1990 Annu. Rev. Phytopathol. 28:451-74 for a review). In some cases, the transgenic plants may also be less susceptible or resistant to viruses which are related to the virus which was the source of the CP gene, but remain susceptible to viruses which are not related.
The potyviruses make up one of the largest groups of plant viruses, and these viruses are responsible for damage to many of the world's major crops. A potyvirus has a (+)sense RNA genome which is usually about 10 kb in length. This is translated into a single polyprotein which is later cleaved and processed into various proteins, including replicases, proteases, movement proteins, and a coat protein. Stark et al., 1989, Bio/Technol. 7:1257-1262 succeeded in producing transgenic tobacco plants which expressed various levels of the CP of a polyvirus, soybean mosaic virus (SMV). The transgenic tobacco plants were challenged with other viruses, including two potyviruses, potato virus Y (PVY) and tobacco etch virus (TEV). It was found that the plants were protected against PVY and TEV, but not unrelated viruses. As SMV, PVY, and TEV show relatively low amino acid homologies, it was concluded that CP-mediated resistance requires some structural as well as some sequence homology.
One of the major vital diseases of corn (Zea mays) is caused by a potyvirus, maize dwarf mosaic virus (MDMV). MDMV occurs throughout the midwestern U.S. corn belt. Symptoms include the appearance of faint yellow stripes on plants 6-7 weeks old and shortened internodes. Eventually, the yellowing leaves may turn purplish or become necrotic. The mature plant may tiller excessively and bear grainless ears. Annual losses due to MDMV infections alone are estimated between 2-10% annual yield. However, if MDMV-A is present in a mixed infection with Maize Chlorotic Dwarf Virus, yields can be decreased by 50%, and a mixed infection of MDMV-B and Maize Chlorotic Mottle Virus, can result in a 100% loss. While one source of genetic resistance to MDMVs has been observed in some lines, such as MDM-1 (McMullen et al. 1989 Mol. Plant-Microbe Interact. 2(6):309-314), the mechanisms promoting the resistance (and their underlying basis) are not well understood.
It would be desirable to identify other genes or introduce numerous sources of resistance into a wide variety of corn plants to protect against the inevitable evolution of the Maize Dwarf Mosaic Virus complex.